Electric system for fuel cell, fuel cell vehicle, and method of supplying electric power

ABSTRACT

An electric system has a fuel cell for generating electric power by being supplied with a reactive gas, an electric storage device having a voltage lower than a voltage output from the fuel cell, a first power supply line connected to the fuel cell, a second power supply line connected to the electric storage device, and a first DC-to-DC converter for performing bidirectional voltage conversion between the first power supply line and the second power supply line. An inverter, a propulsive electric motor, and an air compressor are supplied with electric power from the first power supply line. An electrically operated air conditioner motor, windshield wiper motors, and power window motors, etc., which serve as a second electric accessory, are supplied with electric power from the second power supply line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric system for a fuel cell, afuel cell vehicle, and a method of supplying electric power, and moreparticularly to an electric system having an electric storage devicewhose voltage is lower than the output voltage of a fuel cell and aDC-to-DC converter for performing bidirectional voltage conversionbetween a power supply line connected to the fuel cell and a powersupply line connected to the electric storage device, a fuel cellvehicle incorporating such an electric system, and a method of supplyingelectric power in such an electric system.

2. Description of the Related Art

Recently, fuel cell vehicles carrying fuel cells, which are of excellentfuel efficiency and environment-friendly nature, as propulsive powersources have been developed and put to practical use. Fuel cells for useon fuel cell vehicles are often high-voltage fuel cells because they arerequired to generate large electric power to provide sufficient driveforces to propel the fuel cell vehicles.

Some fuel cell vehicles also carry electric storage devices forassisting in supplying electric power to meet high loads and loadvariations and also for storing regenerated electric power.

If the voltage generated by a fuel cell and the rated voltage of anelectric storage device are different from each other, then a DC-to-DCconverter for performing bidirectional voltage conversion is connectedbetween a power supply line connected to the fuel cell and a powersupply line connected to the electric storage device. The DC-to-DCconverter allows electric power to be efficiently transferred betweenthe fuel cell and the electric storage device and also allowsregenerated electric power to be efficiently stored in the electricstorage device.

Fuel cell vehicles incorporate fuel cell accessories for operating fuelcells, i.e., an air compressor, a hydrogen supply pump, a coolantcirculation pump, etc., in addition to ordinary motor vehicleaccessories including an air conditioner, windshield wipers, powerwindow motors, etc. mounted on general motor vehicles.

One type of connections used for supplying electric power to motorvehicle accessories and fuel cell accessories on fuel cell vehicles isdisclosed in Japanese Laid-Open Patent Publication No. 2004-193063. Asshown in FIG. 16 of the accompanying drawings, the disclosed system hasa power supply line 3 interconnecting a fuel cell 1 and an inverter 2,and power supply lines are branched from the power supply line 3 forsupplying electric power to a motor vehicle accessory 4 and a fuel cellaccessory 5. The power supply line 3 carries a high voltage suitable forenergizing an electric motor. The electric power from the power supplyline 3 is reduced in voltage by a DC-to-DC converter 7 before it issupplied to an electric storage device 6. The DC-to-DC converter 7 iscapable of converting voltages bidirectionally.

Japanese Laid-Open Patent Publication No. 2002-118981 discloses anothersystem of connections as shown in FIG. 17 of the accompanying drawings.As shown in FIG. 17, a power supply line 8 interconnects an electricstorage device 6 and a DC-to-DC converter 7, and power supply lines arebranched from the power supply line 8 for supplying electric power to amotor vehicle accessory 4 and a fuel cell accessory 5. The power supplyline 8 is of a low voltage setting because the voltage carried therebyis reduced by the DC-to-DC converter to a value lower than the voltagecarried by a power supply line 3 interconnecting a fuel cell 1 and aninverter 2.

As described above, many fuel cells mounted on fuel cell vehicles are ofthe high-output type, and fuel cell accessories for operating fuel cellsshould preferably be of the high-output, high-voltage type that iscapable of withstanding high loads.

Windshield wipers, power window motors, etc. as vehicle accessories arenot necessarily required to be of the high-voltage type, but may be ofthe general-purpose, low-voltage type (e.g., 12 V type) for therebyallowing fuel cell electric systems to be constructed inexpensively.

In the systems disclosed in Japanese Laid-Open Patent Publication No.2004-193063 and Japanese Laid-Open Patent Publication No. 2002-118981,since both the vehicle accessory and the fuel cell accessory aresupplied with electric power from the same power supply line, they needto be dedicated accessories compatible with the rated voltage of thepower supply line, or a DC-to-DC converter needs to be connected to atleast one of the vehicle accessory and the fuel cell accessory, makingthe electric system complex.

If a complex system including many accessories is constructed based onthe system revealed in Japanese Laid-Open Patent Publication No.2004-193063, then each of the accessories is connected to thehigh-voltage power supply line 3 interconnecting the fuel cell and theinverter (see FIG. 16). Therefore, these accessories are dedicatedaccessories compatible with the voltage according to the specificationsof the fuel cell or the electric motor, and are hence not for generaluse. Particularly, as electric accessories on other vehicles such asgeneral engine-driven vehicles or hybrid vehicles cannot be used asthose accessories, the complex system is highly costly to construct.

If accessories are to be added to the system disclosed in JapaneseLaid-Open Patent Publication No. 2002-118981, then they are connected tothe low-voltage power supply line 8 that is connected to the electricstorage device 6 shown in FIG. 17. However, the system with the addedaccessories is disadvantageous in that if the DC-to-DC converter failsand the electric power generated by the fuel cell cannot be supplied tothe low-voltage power supply line 8, then the distance that the fuelcell vehicle is able to travel subsequently will be shortened.Specifically, since the fuel cell needs to be supplied with a reactivegas for its operation, it is necessary to operate an air compressor,pumps, etc. for operating the fuel cell. However, as these accessoriesare connected to the low-voltage power supply line, they are energizedby only the electric power which remains in the electric storage device.Consequently, even if a sufficient amount of hydrogen gas is stored inthe fuel tank, since no electric power is obtained from the storedhydrogen gas for actuating the air compressor and the pumps, the fuelcell is shut off at the time the discharge of electric power from theelectric storage device is finished. In addition, inasmuch as the aircompressor and the pumps consume a relatively large amount of electricpower, the electric storage device discharges the stored electric powerat a high rate and cannot be operated continuously for a long period oftime in the event of a failure of the DC-to-DC converter.

If the accessories connected to the low-voltage power supply line 8 inthe system disclosed in Japanese Laid-Open Patent Publication No.2002-118981 are regarded as belonging to a single auxiliary electricsystem, then it is different from the electric systems on vehicles suchas general engine-driven vehicles or hybrid vehicles in that theelectric system includes an air conditioner and a pump. Therefore, theelectric system is not for general use and is highly costly to constructas with the electric system disclosed in Japanese Laid-Open PatentPublication No. 2004-193063.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electric systemwhich allows electric accessories having a plurality of rated voltagesdepending on applications to be used, is of a highly general nature, andcan be constructed inexpensively, a fuel cell vehicle incorporating suchan electric system, and a method of supplying electric power in such anelectric system.

Another object of the present invention is to provide an electric systemincluding a fuel cell and a DC-to-DC converter, which system is capableof operating the fuel cell continuously for a long period of time evenin the event of a failure of the DC-to-DC converter, a fuel cell vehicleincorporating such an electric system, and a method of supplyingelectric power in such an electric system.

An electric system for a fuel cell according to an aspect of the presentinvention has a fuel cell for generating electric power by beingsupplied with a reactive gas, a first power supply line connected to thefuel cell, a first electric accessory serving as at least part of a fuelcell accessory for operating the fuel cell, and an auxiliary electricsystem connected to the first power supply line through a first DC-to-DCconverter for performing bidirectional voltage conversion, the auxiliaryelectric system being operable under a voltage lower than a voltageoutput from the fuel cell, the auxiliary electric system comprising afirst electric storage device, a second power supply line connecting thefirst electric storage device and the first DC-to-DC converter to eachother, and a second electric accessory serving as an electric accessoryother than the fuel cell accessory, the first electric accessory beingconnected for being supplied with electric power from the first powersupply line, and the second electric accessory being connected for beingsupplied with electric power from the second power supply line.

Since the first electric accessory is supplied with electric power fromthe first power supply line and the second electric accessory issupplied with electric power from the second power supply line, thefirst electric accessory and the second electric accessory can be set todifferent voltage specifications depending on applications, allowing theelectric system to have a better energy efficiency and to be constructedinexpensively.

As the auxiliary electric system does not include the first electricaccessory, an electric system on a vehicle other than fuel cell vehiclescan be used as the auxiliary electric system. The electric systemaccording to the present invention is of a better versatility and isinexpensive to construct. Even if the first DC-to-DC converteraccidentally fails to operate, since the first electric accessory iscontinuously supplied with electric power from the first power supplyline, the first electric accessory is not essentially affected by aremaining amount of electric power stored in the electric storagedevice, and the fuel cell can continuously be operated for aconsiderably long period of time.

The first electric accessory serves as at least part of a fuel cellaccessory for operating the fuel cell, and the second electric accessoryserves as an electric accessory other than the fuel cell accessory.Consequently, the fuel cell accessory may be of the high voltage typefor a better energy efficiency, and ordinary vehicle accessories otherthan the fuel cell accessory may be general-purpose accessories,allowing the electric system to be constructed inexpensively.

The first electric accessory may comprise at least one of an aircompressor for supplying air under pressure to the fuel cell, a hydrogenpump for supplying hydrogen to the fuel cell, and a coolant circulationpump for cooling the fuel cell. Since the air compressor, the hydrogensupply pump, and the coolant circulation pump pose a high load, they maybe connected to the first power supply line and actuated under a highvoltage for better energy efficiency.

The electric accessory may have at least a portion connected to thefirst power supply line through a second DC-to-DC converter for loweringa voltage. Generally, as the size of a fuel cell vehicle increases, theoutput power and the drive voltage of a main unit are set to highervalues, and hence the voltage on the first power supply line becomeshigher accordingly. Since the voltage is lowered by the second DC-to-DCconverter to a voltage that is supplied to the first electric accessory,the voltage of the first electric accessory is used commonly regardlessof the vehicle size.

The second DC-to-DC converter may lower a voltage applied thereto andsupply the lowered voltage to the first electric accessory when a loadon a main unit which is supplied with electric power from the firstpower supply line is smaller than a prescribed threshold value, and maydirectly connect input and output terminals thereof and supply electricpower through the input and output terminals to the first electricaccessory when the load on the main unit is equal to or greater than theprescribed threshold value. Inasmuch the input and output terminals ofthe second DC-to-DC converter are directly connected to each other whenthe load on the main unit is high, a switching loss due to a choppingaction of the second DC-to-DC converter is reduced.

The second electric accessory may include at least an electricallyoperated air conditioner motor. The electrically operated airconditioner motor serves as a relatively high load. If an electricallyoperated, general-purpose, low-voltage air conditioner motor isconnected to the first power supply line, then it requires a dedicated,large-capacity DC-to-DC converter, resulting in a larger size. Accordingto the present invention, as the electrically operated air conditionermotor is connected as the second electric accessory to the second powersupply line, it may be of the general-purpose type and may be of a smallsize.

The second electric accessory may include at least a portion, exclusiveof the electrically operated air conditioner motor, connected to thesecond power supply line through a third DC-to-DC converter for loweringa voltage, and also to a second electric storage device. With thisarrangement, the electrically operated air conditioner motor and othervehicle accessories may be set to different voltage specificationsdepending on applications.

An electric system for a fuel cell according to another aspect of thepresent invention has a fuel cell for generating electric power by beingsupplied with a reactive gas, a first electric storage device having avoltage lower than a voltage output from the fuel cell, a first powersupply line connected to the fuel cell, a second power supply lineconnected to the first electric storage device, a first DC-to-DCconverter for performing bidirectional voltage conversion between thefirst power supply line and the second power supply line, a main unitand a first electric accessory for being supplied with electric powerfrom the first power supply line, and a second electric accessory forbeing supplied with electric power from the second power supply line.

A method of supplying electric power according to the present inventionincludes the steps of supplying a main unit and a first electricaccessory with electric power through a first power supply line from afuel cell which generates electric power by being supplied with areactive gas, supplying a second electric accessory with electric powerthrough a second power supply line from an electric storage devicehaving a voltage lower than a voltage output from the fuel cell,supplying electric power having a voltage lowered from a voltage on thefirst power supply line by a DC-to-DC converter capable of performingbidirectional voltage conversion, through the second power supply lineto the second electric accessory or to the electric storage device tocharge the electric storage device, and supplying electric power havinga voltage increased from a voltage on the second power supply line bythe DC-to-DC converter, through the first power supply line to the mainunit or the first electric accessory.

Therefore, the first electric accessory and the second electricaccessory can be set to different voltage specifications depending onapplications, allowing the electric system to have a improved energyefficiency and to be constructed inexpensively.

The method may further include the steps of detecting a failure of theDC-to-DC converter, and limiting response of electric power output fromthe fuel cell if a failure of the DC-to-DC converter is detected in thestep of detecting. Inasmuch as the response of the electric power outputfrom the fuel cell is limited if a failure of the DC-to-DC converter isdetected, a gas shortage in the fuel cell is avoided even if there is noassistive electric current from the electric storage device, so that thefuel cell is prevented from being unduly deteriorated.

On a fuel cell vehicle which is propelled based on the electric powergenerated by the fuel cell in the above electric system, the auxiliaryelectric system connected to the low-voltage power supply line may be ofa general-purpose design and may be constructed inexpensively. The fuelcell vehicle can travel a considerably long distance even when the firstDC-to-DC converter fails to operate.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an electric system for a fuel cellaccording to an embodiment of the present invention;

FIG. 2 is a block diagram of a fuel cell system;

FIG. 3 is a circuit diagram of a first DC-to-DC converter;

FIG. 4 is a circuit diagram of a second DC-to-DC converter;

FIG. 5 is a graph of voltage control characteristics of the secondDC-to-DC converter;

FIG. 6 is a circuit diagram of a power supply switch unit;

FIG. 7 is a simplified block diagram of the electric system;

FIGS. 8 through 10 are a flowchart showing a method of supplyingelectric power;

FIG. 11 is a timing chart of an activating sequence of the electricsystem for the fuel cell;

FIG. 12 is a timing chart of an inactivating sequence of the electricsystem for the fuel cell;

FIG. 13 is a flowchart of a control sequence including a process ofdetecting a failure of a DC-to-DC converter and a process of limitingthe response of electric power output from the fuel cell when a failureof the DC-to-DC converter is detected;

FIG. 14 is a block diagram of an electric system for a fuel cellaccording to a first modification;

FIG. 15 is a block diagram of an electric system for a fuel cellaccording to a second modification;

FIG. 16 is a block diagram of a conventional electric system; and

FIG. 17 is a block diagram of another conventional electric system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An electric system for a fuel cell, a fuel cell vehicle, and a method ofsupplying electric power according to an embodiment of the presentinvention will be described below with reference to FIGS. 1 through 15.

As shown in FIG. 1, the electric system, generally denoted by 10, ismounted on a fuel cell vehicle 12. On fuel cell vehicle 12, electricpower generated by a fuel cell 14 is supplied through an inverter 16 tocontrol a three-phase propulsive electric motor 17 to rotate itsrotatable shaft, causing a gear mechanism 18 including a differentialgear and a transmission connected to the rotatable shaft of the electricmotor 17 to rotate drive wheels 20. The fuel cell 14 comprises alarge-power, high-voltage fuel cell for generating large electric powerto provide sufficient drive forces to propel the fuel cell vehicle 12. Avoltage generated by the fuel cell 14 is herein defined as a voltage V1.

As shown in FIG. 2, a fuel gas supply system 22, a reactive gas supplysystem 24, and a coolant supply system 26 are connected to the fuel cell14. The fuel gas supply system 22 has a hydrogen supply passage 30connected to a hydrogen tank 28 and a fuel gas inlet port 22 a of thefuel cell 14. The hydrogen supply passage 30 is connected to a shutoffvalve 32, an ejector 34, and a hydrogen supply pump 35 that aresuccessively arranged in the order named from the hydrogen tank 28.

The fuel cell 14 has a fuel gas outlet port 22 b connected to a hydrogencirculation passage 36 which is connected to the hydrogen supply passage30 through the ejector 34. A purge valve 40 is connected to a hydrogendischarge passage 38 that is branched from the hydrogen circulationpassage 36.

The reactive gas supply system 24 has an air supply passage 44 connectedto an air compressor (or a supercharger) 42 and a reactive gas inletport 24 a of the fuel cell 14. The fuel cell 14 has a reactive gasoutlet port 24 b connected to an air discharge passage 46 which isconnected to an exhaust pipe 50 through a back pressure valve 49.

The coolant supply system 26 has a coolant circulation passage 52connected to a coolant inlet port 26 a and a coolant outlet port 26 b ofthe fuel cell 14. The coolant circulation passage 52 is connected to acoolant circulation pump 54 for circulating a coolant. The coolantcirculation passage 52 has a heat radiator 53 for radiating heat fromthe coolant to cool the coolant as the coolant is circulated through theheat radiator 53 by the coolant circulation pump 54.

In the fuel cell 14, a hydrogen gas (fuel gas) supplied from thehydrogen supply passage 30 by the hydrogen supply pump 35 and air(reactive gas) compressed to a predetermined pressure by the aircompressor 42 and supplied from the air supply passage 44 are sent torespective electrodes, and consumed by an electrochemical reaction inelectrode catalyst layers to generate electric power. The generatedelectric power is output from a positive output terminal 14 p and anegative output terminal 14 n. In the electric system 10, positive andnegative components provided in pairs are denoted by reference numeralswith a suffix “p” and a suffix “n”, respectively.

An exhaust gas including an unused hydrogen gas in the fuel cell 14 isdischarged from the fuel gas outlet port 22 b into the hydrogencirculation passage 36. The exhaust gas flows through the hydrogencirculation passage 36 and is supplied from the ejector 34 into thehydrogen supply passage 30, and is supplied again as the fuel gas to thefuel cell 14. The air that is consumed in the fuel cell 14 is dischargedfrom the reactive gas outlet port 24 b into the air discharge passage46.

The fuel cell system for operating the fuel cell 14 is not limited tothe above structure. The fuel cell system may be a system free of thehydrogen supply pump 35 or a system wherein the hydrogen supply pump 35,the air compressor 42, and the coolant circulation pump 54 may beoperated by a single electric motor.

Electric motors 35 a, 42 a, 54 a, which serve as a fuel cell accessory,operate the hydrogen supply pump 35, the air compressor 42, and thecoolant circulation pump 54, respectively. The electric motors 35 a, 42a, 54 a are electrically connected to a first power supply line 72 (seeFIG. 1) by electric connections that are omitted from illustration inFIG. 2.

As shown in FIG. 1, the electric system 10 has an electric storagedevice (first electric storage device) 70 for storing electric powerunder a voltage lower than the output voltage of the fuel cell 14, afirst power supply line 72 connected to the output terminals 14 p, 14 nof the fuel cell 14, a second power supply line 73 connected to outputterminals 70 p, 70 n of the electric storage device 70, and a firstDC-to-DC converter 74 for converting voltages bidirectionally betweenthe first power supply line 72 and the second power supply line 73. Thefirst power supply line 72 comprises a positive line 72 p and a negativeline 72 n, and the second power supply line 73 comprises a positive line73 p and a negative line 73 n. The electric storage device 70 maycomprise any of various secondary battery, a lead battery, a lithium ionbattery, an electric double-layer capacitor, or the like.

The electric system 10 has a first electric accessory 76 for beingsupplied with electric power from the first power supply line 72 and asecond electric accessory 78 for being supplied with electric power fromthe second power supply line 73. The first electric accessory 76 is afuel cell accessory for operating the fuel cell 14. The second electricaccessory 78 is an electric accessory other than the fuel cellaccessory, and is of a general accessory for a vehicle mounted on ageneral motor vehicle other than the fuel cell vehicle 12.

Components of the electric system 10 which are shown in FIG. 1 as beingpositioned below the first DC-to-DC converter 74, i.e., the electricstorage device 70, a power supply switch unit 98, the second powersupply line 73, and the second electric accessory 78, jointly make up anauxiliary electric system 100 as a single unit, which is connected tothe first DC-to-DC converter 74 through a terminal 101. However, theauxiliary electric system 100 is not limited to being assembled as asingle unit, but may be a conceptual system in the form of a circuitthat is connected through the first DC-to-DC converter 74 to the firstpower supply line 72 and operable under a voltage V2 (or a voltage V4)lower than the output voltage V1 of the fuel cell 14, the circuitincluding the electric storage device 70, the second power supply line73, and the second electric accessory 78.

The inverter 16 and the propulsive electric motor 17 are included in theelectric system 10, and are supplied with electric power from the firstpower supply line 72. Of the electric devices mounted on the fuel cellvehicle 12, the propulsive electric motor 17 presents a particularlyhigh load and serves as a major component on the self-propelled vehicle.The propulsive electric motor 17 may be referred to as a principalcomponent against the first electric accessory 76 and the secondelectric accessory 78. The inverter 16 converts DC electric powersupplied from the first power supply line 72 into three-phase ACelectric power, and supplies the three-phase AC electric power at afrequency and a power level which are commensurate with the vehicledriver's action on the accelerator pedal, etc.

The first power supply line 72 has a voltage sensor 72 a for measuringthe voltage V1. The positive line 72 p and the negative line 72 n haverespective main contactors 77 p, 77 n, and the negative line 72 n has areverse-current blocking diode 72 b.

As shown in FIG. 3, the first DC-to-DC converter 74 has a voltage sensor74 a, a protective resistor 74 b, and a stabilizing capacitor 74 c whichare connected to a higher-voltage side, i.e., the first power supplyline 72, and a voltage sensor 74 d, a protective resistor 74 e, and astabilizing capacitor 74 f which are connected to a lower-voltage side,i.e., the second power supply line 73. The first DC-to-DC converter 74also has a voltage-increasing switching device 74 g and avoltage-decreasing switching device 74 h.

The first DC-to-DC converter 74 has a pair of lines 83 p, 83 n connectedto the higher-voltage side, and a pair of lines 85 p, 85 n connected tothe lower-voltage side. The negative lines 83 n, 85 n are connected toeach other. The switching device 74 g and the switching device 74 h areconnected in series to each other, the voltage-increasing switchingdevice 74 g being connected to the line 83 n and the voltage-decreasingswitching device 74 h to the line 83 p. The low-voltage line 85 p isconnected to a branched point between the voltage-increasing switchingdevice 74 g and the voltage-decreasing switching device 74 h through areactor 74 i for stabilizing an electric current. The high-voltage lines83 p, 83 n are connected respectively to the positive line 72 p and thenegative line 72 n of the first power supply line 72. The low-voltagelines 85 p, 85 n are connected respectively to the positive line 73 pand the negative line 73 n of the second power supply line 73.

The switching devices 74 g, 74 h operate as choppers that are repeatedlyturned on and off at a high frequency for thereby reducing the voltageV1 of electric power supplied from the first power supply line 72 to thevoltage V2. Therefore, the first DC-to-DC converter 74 supplies electricpower of the voltage V2 to the second power supply line 73. The electricpower thus supplied from the first power supply line 72 through thefirst DC-to-DC converter 74 to the second power supply line 73 isapplied to charge the electric storage device 70 or supplied to thesecond electric accessory 78. Each of the switching devices 74 g, 74 hand switching devices 79 d, 90 d, which are to be described later,comprises a semiconductor device such as a transistor, a thyristor, anFET (Field Effect Transistor), an IGBT (Insulated Gate BipolarTransistor), or the like.

When the propulsive electric motor 17 is put under a high load, thefirst DC-to-DC converter 74 increases the voltage V2 of electric powerfrom the second power supply line 73 to the voltage V1, and supplieselectric power of the voltage V1 to the first power supply line 72. Atthis time, the electric power on the second power supply line 73 issupplied from the electric storage device 70 as it is discharged.

As described above, the first electric accessory 76 is a fuel cellaccessory for operating the fuel cell 14. The first electric accessory76 includes the electric motor 42 a for operating the air compressor 42,the electric motor 35 a for operating the hydrogen supply pump 35, andthe electric motor 54 a for operating the coolant circulation pump 54.Of these electric motors, the electric motor 42 a for operating the aircompressor 42 is supplied with electric power from the first powersupply line 72 through a second DC-to-DC converter 79. The firstelectric accessory 76 may comprise a fuel reformer.

As shown in FIG. 4, the second DC-to-DC converter 79 has a function tolower the voltage V1 of electric power on the first power supply line 72to a voltage V3. The second DC-to-DC converter 79 comprises a voltagesensor 79 a, a protective resistor 79 b, a stabilizing capacitor 79 c, aswitching device 79 d, a reactor 79 e for stabilizing an electriccurrent, and a surge-cutoff diode 79 f. The electric motor 42 acomprises an AC three-phase electric motor whose rotation is controlledby an inverter 81.

FIG. 5 is an IV characteristic diagram having a horizontal axisrepresenting electric currents and a vertical axis representingvoltages. FIG. 5 shows voltage characteristics of the fuel cell 14 incomparison with the voltage V3 that is controlled by the switchingdevice 79 d. When a load current I (or a load L) of the propulsiveelectric motor 17 is smaller than a prescribed threshold valve Ls, theswitching device 79 d operates as a chopper to lower the voltage V1applied thereto to the voltage V3. When the load L is equal to orgreater than the prescribed threshold valve Ls, the switching device 79d is continuously turned on to directly connect input and outputterminals thereof, supplying electric power to the inverter 81 and theair compressor 42. When the input and output terminals of the switchingdevice 79 d are directly connected to each other, it generates lessheat, allowing the second DC-to-DC converter 79 to be small in size.

As described above, the second electric accessory 78 is an electricaccessory other than the fuel cell accessory, and represents ordinaryvehicle accessories including an electrically operated air conditionermotor 80, windshield wiper motors 82, and power window motors 84, etc.

The electrically operated air conditioner motor 80 is a three-phaseelectric motor whose rotation is controlled by an inverter 86. Since thefuel cell vehicle 12 has no internal combustion engine mounted thereon,there is no rotational drive source that rotates at all times.Therefore, the fuel cell vehicle 12 has an electrically operated airconditioner, with a compressor 88 being operated by the electricallyoperated air conditioner motor 80. As no internal combustion engine ismounted on the fuel cell vehicle 12, the fuel cell vehicle 12 does nothave a heat source for generating a large amount of heat. Theelectrically operated air conditioner is also used to heat the space inthe passenger cabin of the fuel cell vehicle 12, and operates as aso-called heat pump. The electrically operated air conditioner motor 80compresses and circulates a refrigerant by operating the compressor 88thereby to adjust the temperature in the passenger cabin. Theelectrically operated air conditioner motor 80 is of a relatively largecapacity as it needs to compress the refrigerant.

The windshield wiper motors 82 and power window motors 84, etc., otherthan the electrically operated air conditioner motor 80, of the secondelectric accessory 78 are connected to the second power supply line 73through a third DC-to-DC converter 90 which reduces the voltage V2 to alower voltage V4 (e.g., 12 V). The third DC-to-DC converter 90 hasoutput lines to which a lead battery 92 (second electric storage device)is connected.

The third DC-to-DC converter 90 operates to lower a voltage in the samemanner as the second DC-to-DC converter 79. Specifically, the thirdDC-to-DC converter 90 has switching devices that operate as choppers tolower the applied voltage V2 to the voltage V4 and output the voltageV4.

The electric storage device 70 is connected to the second power supplyline 73 through the power supply switch unit 98 which serves to controlconnections. As shown in FIG. 6, the power supply switch unit 98 has avoltage sensor 103 for detecting the voltage V2, a pair of batterycontactors 102 p, 102 n connected respectively to the positive line 73 pand the negative line 73 n, and a series-connected circuit of aprecontactor 104 and a limiting resistor 106 which are connected inparallel to the battery contactor 102 p.

The main contactors 77 p, 77 n, the first DC-to-DC converter 74, thesecond DC-to-DC converter 79, and the power supply switch unit 98 areconnected to a power supply controller 110, and perform a predeterminedpower supply controlling process under the control of the power supplycontroller 110. The third DC-to-DC converter 90 is not connected to thepower supply controller 110, and converts voltages by itself.

The power supply controller 110 has a CPU (Central Processing Unit) as amain control unit, a RAM (Random Access Memory) and a ROM (Read OnlyMemory) as a storage unit, and a drive. The CPU reads a program andexecutes the program in cooperation with the storage unit, etc. toperform the power supply controlling process. The power supplycontroller 110 is supplied with electric power from the lead battery 92,for example, and hence can operate even in the event of a shutdown ofthe fuel cell 14.

Electric motors 35 a, 42 a, 54 a, which serve as the fuel cellaccessory, are basically supplied with electric power from the firstpower supply line 72. Other fuel cell accessories, such as a controller,a sensor, etc. (not shown), which consume a small amount of electricpower may be supplied with electric power from the second power supplyline 73.

The electric system 10 thus constructed is illustrated in simplifiedblock form in FIG. 7. As shown in FIG. 7, the voltages on the firstpower supply line 72 and the second power supply line 73 arebidirectionally converted by the first DC-to-DC converter 74 in theelectric system 10. The first electric accessory 76 as a fuel cellaccessory for operating the fuel cell 14 is supplied with electric powerbranched from the first power supply line 72, and the second electricaccessory 78 as an electric accessory other than the fuel cell accessoryis supplied with electric power branched from the second power supplyline 73. The auxiliary electric system 100 is connected to the firstpower supply line 72 through the DC-to-DC converter 74, and has no otherconnections, so that the auxiliary electric system 100 is asemiindependent system in terms of circuitry.

Hybrid electric vehicles (HEVs) which have been developed and put topractical use in recent years have an engine and an electric motor aspropulsive power sources, and an electric storage device, such as theelectric storage device 70, for efficiently energizing the electricmotor. Hybrid electric vehicles also have an electrically operated airconditioner motor independent of the operation of the engine in order toair-condition the passenger cabin while the hybrid electric vehicle isbeing driven by the electric motor with the engine being shut off.Hybrid electric vehicles further have ordinary vehicle accessoriesincluding the windshield wiper motors 82 and the power window motors 84,etc.

Stated otherwise, most components of the electric system on a hybridelectric vehicle are similar to those of the auxiliary electric system100 of the electric system 10 according to the present embodiment.Actually, the auxiliary electric system 100 can be used in common withthe electric systems on the fuel cell vehicle 12 and the hybrid electricvehicle, and hence is of a high versatility and high compatibility withthose electric systems. Therefore, the auxiliary electric system 100offers advantages of scale and is inexpensive to construct.

Operation of the electric system 10 in the fuel cell vehicle 12 thusconstructed will be described below with reference to FIGS. 8 through12. A processing sequence, which represents a method of supplyingelectric power, shown in FIGS. 8 through 10 is software-implementedmainly by the power supply controller 110 and a controller of the fuelcell 14 together. In FIGS. 11 and 12, the higher level in each of thecomponent operation statuses represents a turned-on or activated stateand the lower level represents a turned-off or inactivated state, exceptthat it represents an increased voltage for the operation of the firstDC-to-DC converter 74. It is assumed in the description which followsthat time elapses in the order of suffix numbers of times t1, t2, t3, .. . .

In step S1 shown in FIG. 8, the power supply switch is turned on, andthe power supply controller 110 is activated and performs apredetermined initializing process. The initializing process includes asecurity check, a shift position check, a rotational speed check on thepropulsive electric motor 17, an initialization of the RAM, etc.

In step S2, the precontactor 104 is turned on to precharge the positiveline 73 p of the second power supply line 73 at time t1 when theinitializing process is terminated.

In step S3, the battery contactor 102 n on the negative line 73 n of thesecond power supply line 73 is turned on at time t2. Now, electric poweris supplied from the electric storage device 70 to the second powersupply line 73. Because of the limiting resistor 106, the electriccurrent supplied through the second power supply line 73 is limited,protecting the contacts of the battery contactor 102 n. In steps S2, S3,since the voltage V2 is applied to the second power supply line 73, thevoltage V2 is also applied through the first DC-to-DC converter 74 tothe first power supply line 72 to precharge the first DC-to-DC converter74.

In step S4, the battery contactor 102 p on the positive line 73 p of thesecond power supply line 73 is turned on at time t3. A sufficient amountof electric power depending on the charged quantity of the electricstorage device 70 is now supplied to the second power supply line 73. Atthis time, since the second power supply line 73 has been precharged,the voltage V2 remains almost unchanged, protecting the contacts of thebattery contactor 102 p. At a suitable time after the time t3, theprecontactor 104 is turned off.

In step S5, the first DC-to-DC converter 74 starts to increase thevoltage on the second power supply line 73 at time t4. The voltage onthe first power supply line 72 gradually increases until it reaches thevoltage V2 at time t5. At time t5, the second DC-to-DC converter 79starts operating, starting to supply electric power to the aircompressor 42.

The third DC-to-DC converter 90 operates by itself to start convertingthe voltage at this time, supplying electric power to the ordinaryvehicle accessories including the windshield wiper motors 82 and thepower window motors 84, etc.

In step S6, the main contactors 77 p, 77 n on the first power supplyline 72 are turned on at time t5. At this time, the voltage V1 hasalready been applied to the first power supply line 72 by the voltageincreasing action of the first DC-to-DC converter 74. Inasmuch as thefuel cell 14 has been shut off, no large electric current does not flowabruptly through the main contactors 77 p, 77 n, so that the contacts ofthe main contactors 77 p, 77 n are protected. At time t5, the secondDC-to-DC converter 79 starts operating, starting to supply electricpower to the motor 42 a of the air compressor 42.

In step S7, the air compressor 42 is actuated by the controller of thefuel cell 14, the back pressure valve 49 starts controlling the backpressure at time t6. Thereafter, the shutoff valve 32 is opened and thehydrogen supply pump 35 is actuated to supply a suitable amount ofhydrogen gas to the fuel cell 14. The fuel cell 14 now begins togenerate electric power, and supply electric power under the voltage V1from the output terminals 14 p, 14 n to the first power supply line 72.The coolant circulation pump 54 is actuated to circulate the coolant tocool the fuel cell 14 to a temperature suitable for generating electricpower.

The electric system 10 is activated in the manner described above, andthereafter enters a normal mode of operation.

In the normal mode of operation (see FIG. 9), a load L on the propulsiveelectric motor 17 is detected based on a voltage value detected by thevoltage sensor 72 a and a predetermined signal obtained from theinverter 16, and it is determined whether the load L is smaller than aprescribed threshold value Ls or not in step S8. If L<Ls, then controlgoes to step S10, and if L≧Ls, then control goes to step S11.

In step S10 (L<Ls), the power supply controller 110 instructs the secondDC-to-DC converter 79 to perform chopping operation, lowering thevoltage V1 on the first power supply line 72 to the voltage V3 andapplying the voltage V3 to the inverter 81. Thereafter, control goes tostep S12.

In step S11 (L≧Ls), the power supply controller 110 instructs the secondDC-to-DC converter 79 to directly connect its input terminals to itsoutput terminals, i.e., keeps its switching devices turned on. At thistime, the voltage on the first power supply line 72 has been made lowerthan the voltage V1, and the lower voltage is directly applied to theinverter 81. Thus, a switching loss caused by the chopping operation ofthe second DC-to-DC converter 79 is eliminated.

In step S12, a voltage lowering condition for the first DC-to-DCconverter 74 is determined. Specifically, the voltage lowering conditionis satisfied if the second electric accessory 78 consumes a large amountof electric power or a charged quantity SOC of the electric storagedevice 70 is less than a predetermined value and also if the fuel cell14 has an extra current outputting capability. If the voltage loweringcondition is satisfied, then control goes to step S13, and if thevoltage lowering condition is not satisfied, then control goes to stepS14. The voltage lowering condition and a voltage increasing conditionin step S14 are determined based on output signals from the voltagesensors 74 a, 74 d and a sensor on the electric storage device 70.

In step S13, the power supply controller 110 instructs the firstDC-to-DC converter 74 to lower the voltage V1 on the first power supplyline 72 to the voltage V2, and supply electric power under the voltageV2 to the second electric accessory 78 through the second power supplyline 73 or to charge the electric storage device 70. Thereafter, controlgoes to step S17.

Because of the processing in step S13, either electric storage device 70is charged for use as a power supply or the second electric accessory 78is supplied with electric power from the fuel cell 14 to make up for anelectric power shortage from the electric storage device 70.

In step S14, a voltage increasing condition for the first DC-to-DCconverter 74 is determined. Specifically, the voltage increasingcondition is satisfied if the first electric accessory 76 such as theelectric motor 42 a or the like and the propulsive electric motor 17consume a large amount of electric power or the output current from thefuel cell is equal to or greater than a prescribed maximum value andalso if the charged quantity SOC of the electric storage device 70 isequal to or greater than the predetermined value. If the voltageincreasing condition is satisfied, then control goes to step S15, and ifthe voltage increasing condition is not satisfied, then control goes tostep S16.

In step S15, the power supply controller 110 instructs the firstDC-to-DC converter 74 to increase the voltage V2 on the second powersupply line 73 to the voltage V1, and supply electric power under thevoltage V1 to the propulsive electric motor 17 and the first electricaccessory 76 through the first power supply line 72. Thereafter, controlgoes to step S17.

Because of the processing in step S15, if the propulsive electric motor17 needs to be energized with a large amount of electric current as whenthe fuel cell vehicle 12 is to be quickly accelerated, but the fuel cell14 alone fails to supply electric power for such a quick acceleration,then the electric storage device 70 supplies supplementary electricpower.

In step S16, both the voltage lowering action and the voltage increasingaction of the first DC-to-DC converter 74 are stopped. At this time, noelectric power conversion between the first power supply line 72 and thesecond power supply line 73 is performed. The propulsive electric motor17 and the first electric accessory 76 are supplied with electric powerfrom the fuel cell 14 as a sole power supply, and the second electricaccessory 78 is supplied with electric power from the electric storagedevice 70 and the lead battery 92 which act as a power supply.

In step S17, the state of the power supply switch is confirmed. If thepower supply switch remains turned on, then control goes back to step S8to continue the normal mode of operation. If the power supply switch isturned off, then control goes to step S18 (see FIG. 10) for a shutdownprocess for the electric system 10.

In step S18 of the shutdown process (see FIGS. 10 and 12), the shutoffvalve 32 is turned off to stop supplying hydrogen at time t7 when theturned-off state of the power supply switch is recognized.

In step S19, the air compressor 42, the hydrogen supply pump 35, and thecoolant circulation pump 54 are turned off by the controller of the fuelcell 14, and the shutoff valve 32 is closed to stop supplying thehydrogen gas to the fuel cell 14 at time t8. The fuel cell 14 now stopsgenerating electric power.

In step S20, the second DC-to-DC converter 79 is shut off to stopsupplying electric power to the inverter 81 at time t9. At this time,the first DC-to-DC converter 74 lowers the voltage to discharge electriccharges that are left in the fuel cell 14, gradually lowering thevoltage on the first power supply line 72. The discharged electriccharges are at least partly charged in the electric storage device 70.

In step S21, the battery contactor 102 p is turned off at time t10 whenthe voltage on the first power supply line 72 becomes lower than thevoltage that is suitable for charging the electric storage device 70.The electric storage device 70 now finishes being charged. The batterycontactor 102 n is kept turned on. The first DC-to-DC converter 74continues its voltage lowering action, further discharging residualelectric charges from the fuel cell 14. The residual electric chargesare charged into the lead battery 92 through the third DC-to-DCconverter 90, or consumed by the power supply controller 110, etc., ordie out in course of time by way of a small electric current leakage.

In step S22, the main contactor 77 p is turned off at time t11 when thevoltage on the first power supply line 72 is lowered to a sufficientlysmall value. The first power supply line 72 is now disconnected from theoutput terminal 14 p of the fuel cell 14, and the voltage on the firstpower supply line 72 is eliminated. The first DC-to-DC converter 74 andthe third DC-to-DC converter 90 are shut off, and the voltage on thesecond power supply line 73 is eliminated. If the voltages on the firstpower supply line 72 and the second power supply line 73 remainessentially unchanged, then it is recognized that the contacts of themain contactor 77 p suffer a failure, and a predetermined error processis carried out.

In step S23, the main contactor 77 n and the battery contactor 102 n areturned off at time t12, completely disconnecting the fuel cell 14 andthe electric storage device 70 respectively from the first power supplyline 72 and the second power supply line 73. Thereafter, a power holdingmode in a power self-holding circuit which supplies electric power fromthe lead battery 92 to the power supply controller 110 is canceled, thusde-energizing the power supply controller 110.

With the electric system 10, the fuel cell vehicle 12, and the method ofsupplying electric power according to the present embodiment, asdescribed above, the first electric accessory 76 is supplied withelectric power from the first power supply line 72 and the secondelectric accessory 78 is supplied with electric power from the secondpower supply line 73. Therefore, the first electric accessory 76 and thesecond electric accessory 78 can be set to different voltagespecifications depending on applications, allowing the electric system10 to have a improved energy efficiency and to be constructedinexpensively.

The air compressor 42 as the first electric accessory 76 is part of thefuel cell accessory for operating the fuel cell 14, and the secondelectric accessory 78 is an electric accessory other than the fuel cellaccessory. Consequently, the fuel cell accessory may be of the highvoltage type for improved energy efficiency, and the second electricaccessory 78 may be an ordinary general-purpose vehicle accessory, sothat the electric system 10 can be of a lower cost.

The first electric accessory 76 as the fuel cell accessory for operatingthe fuel cell 14 is supplied with electric power from the first powersupply line 72, and is not supplied with electric power directly fromthe second power supply line 73 without the first DC-to-DC converter 74interposed therebetween. Therefore, even if the first DC-to-DC converter74 accidentally fails to perform its voltage increasing function orvoltage lowering function, the first electric accessory 76 is suppliedwith electric power from the fuel cell 14 through the first power supplyline 72, allowing the fuel cell vehicle 12 to continue traveling alonger distance or a longer period of time.

In this case, the electric storage device 70 and the lead battery 92 arenot charged. However, since the second electric accessory 78 consumesless electric power than the first electric accessory 76, the electricstorage device 70 and the lead battery 92 are discharged at a limitedrate and keep operating for a considerably long period of time. Even ifthe charged quantities of the electric storage device 70 and the leadbattery 92 are reduced to the extent that the second electric accessory78 is no longer energized, the first electric accessory 76, thepropulsive electric motor 17, etc. keep operating with the electricpower supplied from the fuel cell 14, so that the fuel cell vehicle 12continues to travel. Stated otherwise, the fuel cell vehicle 12 requiresno special measures for continuing traveling or enabling itself totravel for a predetermined distance in an accidental situation where thefirst DC-to-DC converter 74 fails to operate. The electric storagedevice 70 does not necessarily supply electric power to the firstelectric accessory 76 to keep the fuel cell vehicle 12 traveling.Therefore, basically, the electric storage device 70 may be of a smallcapacity that is only large enough to energize the second electricaccessory 78 and also to actuate the air compressor 42 when it starts tooperate.

Since the air compressor 42 as the first electric accessory 76, thehydrogen supply pump 35, and the coolant circulation pump 54 serve as ahigh load, they are connected to the first power supply line 72 carryingthe voltage V1 and are actuated under the high voltage for a betterenergy efficiency.

Generally, as the size of the fuel cell vehicle 12 increases, the outputpower and the drive voltage of the propulsive electric motor 17 are setto higher values, and hence the voltage on the first power supply line72 becomes higher. In the electric system 10, since the voltage V1 islowered by the second DC-to-DC converter 79 to a voltage that issupplied to the first electric accessory 76, the voltage of the firstelectric accessory 76 is used commonly regardless of the vehicle size.Accordingly, the fuel cell vehicle 12 can be designed in a shorterperiod of time, and can be manufactured at a lower cost and withincreased productivity.

The second electric accessory 78 includes the electrically operated airconditioner motor 80 which serves as a relatively high load, and theelectrically operated air conditioner motor 80 is supplied with electricpower from the second power supply line 73. Therefore, the electricallyoperated air conditioner motor 80 is not affected by voltagespecifications of the first power supply line 72 which carries adifferent voltage depending on the vehicle size, and the fuel cellvehicle 12 does not need a large-capacity DC-to-DC converter forconnecting the electrically operated air conditioner motor 80 to thefirst power supply line 72.

Inasmuch as the windshield wiper motors 82 and the power window motors84, etc. are connected to the second power supply line 73 through thethird DC-to-DC converter 90, the voltage for the electrically operatedair conditioner motor 80 and the voltages for the other vehicleaccessories can be set to different values, so that the electricallyoperated air conditioner motor 80 can be set to a suitable voltagespecification depending on the load thereon. Furthermore, the windshieldwiper motors 82 and the power window motors 84, etc. are set to a lowvoltage (e.g., 12 V), and hence can be ordinary general-purpose vehicleaccessories. When they are connected to the lead battery 92, theiroperation is stabilized.

A process of detecting a failure of the first DC-to-DC converter 74 andtaking a countermeasure against the failure will be described below withreference to FIG. 13. The sequence shown in FIG. 13 is carried out bythe power supply controller 110 in given small time cycles.

In step S101, a given failure detector determines whether there is aconverter failure or not. If a converter failure is detected, thencontrol goes to step S102. If no converter failure is detected, thencontrol jumps to step S105. A converter failure is judged by detectingelectric currents at the input and output sides of the first DC-to-DCconverter 74 and determining whether a proper electric current flowsthrough the first DC-to-DC converter 74 or not. If the first DC-to-DCconverter 74 has a function to output a failure signal in the event of aconverter failure, then a converter failure may be judged based on thefailure signal.

In step S102, the electric power consumed by the electrically operatedair conditioner motor 80 is limited or the electrically operated airconditioner motor 80 is turned off. The electrically operated airconditioner motor 80 is connected closer to the electric storage device70 than the first DC-to-DC converter 74 (see FIG. 7). Of fuel cellaccessories for operating the fuel cell 14, the fuel cell controller206, valves, a radiator fan, etc., which consume a small amount ofelectric power, are generally connected to the second power supply line73 which is of a voltage lower than the first power supply line 72. Whenthe electric power consumed by the electrically operated air conditionermotor 80 is limited, the electric power discharged from the electricstorage device 70 is limited, increasing the operating times of thesefuel cell accessories which consume a small amount of electric power. Asa result, the operating time of the fuel cell 14 is increased, allowingthe fuel cell vehicle 12 to travel a longer distance.

In step S103, a regeneration limiting feedforward process is performed.Specifically, a failure regenerative electric power limit value isselectively set to a predetermined value for a process in step S105.

In step S104, a motor torque response limiting process is performed.Specifically, a motor torque command is limited based on an allowabletorque change based on the vehicle speed. By thus limiting the responseof the propulsive electric motor 17, the response of the electric poweroutput from the fuel cell 14 is also limited. The loads on the fuel cell14 include the propulsive electric motor 17, the hydrogen supply pump35, the air compressor 42, and the coolant circulation pump 54. Of theseloads, the propulsive electric motor 17 is the largest. Therefore, theresponse of the electric power output from the fuel cell 14 caneffectively be limited by limiting the torque of the propulsive electricmotor 17.

In step S105, a regeneration limiting feedback process is performed.Specifically, a maximum regenerative amount of the propulsive electricmotor 17 is limited based on the difference between a system upper limitvoltage and the voltage V1 for thereby preventing the voltage V1 on thefirst power supply line 72 from increasing excessively.

In step S106, a motor output limiting process is performed.Specifically, an allowable motor output electric power upper limit valueis established based on the difference between a smaller one of theamount of electric power that can be generated by the fuel cell 14 andthe amount of electric power required by the load system, and the amountof electric power generated by the fuel cell 14. According to the motoroutput limiting process, the output of the propulsive electric motor 17is appropriately limited. After step S106, the control sequence shown inFIG. 13 is finished in the present cycle.

Generally, the supplied amounts of the fuel gas and the reactive gas inthe fuel cell 14 are established depending on the generated amount ofelectric power. A certain limitation is posed on a quick change in thesupplied amounts of the fuel gas and the reactive gas because of theinertia of the fluid, piping resistances, and air compressor dynamiccharacteristics, possibly causing the system to fail to catch up on arequired change in the electric power, i.e., to fail to be sufficientlyresponsive. If the first DC-to-DC converter 74 suffers a failure, as noassistive electric current is supplied from the electric storage device70, the fuel cell 14 needs to generate electric power in excess of anallowable change in electric power, resulting in insufficientcapabilities of the pumps for supplying the fuel gas and the reactivegas. As a consequence, the fuel cell operates in gas shortages, andtends to be deteriorated soon.

Since the sequence shown in FIG. 13 includes the process of detecting afailure of the first DC-to-DC converter 74 and the process of limitingthe response of the electric power output from the fuel cell 14 if afailure of the first DC-to-DC converter 74 is detected, a gas shortagein the fuel cell 14 is avoided even if there is no assistive electriccurrent from the electric storage device 70, so that the fuel cell 14 isprevented from being unduly deteriorated. Because the response islimited, the amount of a fuel gas that is consumed is reduced, allowingthe fuel cell vehicle 12 to travel a longer distance.

The electric system 10 may be modified depending on the makeup of thefirst electric accessory 76, as follows:

FIG. 14 shows in block form an electric system 10 a according to a firstmodification. In the electric system 10 a, the air compressor 42, thecoolant circulation pump 54, and the hydrogen supply pump 35 of thefirst electric accessory 76 are of voltage specifications correspondingto the voltage V1 on the first power supply line 72, and are directlyconnected to the first power supply line 72 without any voltage-loweringDC-to-DC converter interposed therebetween.

FIG. 15 shows in block form an electric system 10 b according to asecond modification. In the electric system 10 b, the air compressor 42,the coolant circulation pump 54, and the hydrogen supply pump 35 are ofindividual voltage specifications corresponding to respective loads, andare supplied with electric power from respective DC-to-DC converters79A, 79B, 79C which converts the supplied voltage into respectivevoltages for the air compressor 42, the coolant circulation pump 54, andthe hydrogen supply pump 35.

As with the electric system 10, the electric systems 10 a, 10 b areversatile and are inexpensive to construct, and can continuously operatethe fuel cell 14 for a considerably long period of time even in theevent of a failure of the first DC-to-DC converter 74.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. An electric system for a fuel cell, comprising: a fuel cell forgenerating electric power by being supplied with a reactive gas; a firstpower supply line connected to said fuel cell; a first electricaccessory serving as at least part of a fuel cell accessory foroperating said fuel cell; and an auxiliary electric system connected tosaid first power supply line through a first DC-to-DC converter forperforming bidirectional voltage conversion, said auxiliary electricsystem being operable under a voltage lower than a voltage output fromsaid fuel cell; said auxiliary electric system comprising: a firstelectric storage device; a second power supply line connecting saidfirst electric storage device and said first DC-to-DC converter to eachother; and a second electric accessory serving as an electric accessoryother than said fuel cell accessory; said first electric accessory beingconnected for being supplied with electric power from said first powersupply line; and said second electric accessory being connected forbeing supplied with electric power from said second power supply line.2. An electric system according to claim 1, wherein said first electricaccessory comprises at least one of an air compressor for supplying airunder pressure to said fuel cell, a hydrogen pump for supplying hydrogento said fuel cell, and a coolant circulation pump for cooling said fuelcell.
 3. An electric system according to claim 1, wherein said firstelectric accessory has at least a portion connected to said first powersupply line through a second DC-to-DC converter for lowering a voltage.4. An electric system according to claim 3, wherein said second DC-to-DCconverter lowers a voltage applied thereto and supplies the loweredvoltage to said first electric accessory when a load on a main unitwhich is supplied with electric power from said first power supply lineis smaller than a prescribed threshold value, and directly connectsinput and output terminals thereof and supplies electric power throughthe input and output terminals to said first electric accessory when theload on said main unit is equal to or greater than said prescribedthreshold value.
 5. An electric system according to claim 1, whereinsaid second electric accessory includes at least an electricallyoperated air conditioner motor.
 6. An electric system according to claim5, wherein said second electric accessory includes at least a portion,exclusive of said electrically operated air conditioner motor, connectedto said second power supply line through a third DC-to-DC converter forlowering a voltage, and also to a second electric storage device.
 7. Anelectric system for a fuel cell, comprising: a fuel cell for generatingelectric power by being supplied with a reactive gas; a first electricstorage device having a voltage lower than a voltage output from saidfuel cell; a first power supply line connected to said fuel cell; asecond power supply line connected to said first electric storagedevice; a first DC-to-DC converter for performing bidirectional voltageconversion between said first power supply line and said second powersupply line; a main unit and a first electric accessory for beingsupplied with electric power from said first power supply line; and asecond electric accessory for being supplied with electric power fromsaid second power supply line.
 8. A method of supplying electric power,comprising the steps of: supplying a main unit and a first electricaccessory with electric power through a first power supply line from afuel cell which generates electric power by being supplied with areactive gas; supplying a second electric accessory with electric powerthrough a second power supply line from an electric storage devicehaving a voltage lower than a voltage output from said fuel cell;supplying electric power having a voltage lowered from a voltage on saidfirst power supply line by a DC-to-DC converter capable of performingbidirectional voltage conversion, through said second power supply lineto said second electric accessory or to said electric storage device tocharge the electric storage device; and supplying electric power havinga voltage increased from a voltage on said second power supply line bysaid DC-to-DC converter, through said first power supply line to saidmain unit or said first electric accessory.
 9. A method according toclaim 8, wherein said first electric accessory comprises at least one ofan air compressor for supplying air under pressure to said fuel cell, ahydrogen pump for supplying hydrogen to said fuel cell, and a coolantcirculation pump for cooling said fuel cell.
 10. A method according toclaim 8, wherein said first electric accessory has at least a portionconnected to said first power supply line through a second DC-to-DCconverter for lowering a voltage.
 11. A method according to claim 10,further comprising the steps of: lowering a voltage applied to saidsecond DC-to-DC converter and supplying the lowered voltage to saidfirst electric accessory when a load on said main unit is smaller than aprescribed threshold value; and directly connecting input and outputterminals of said second DC-to-DC converter and supplying electric powerthrough the input and output terminals to said first electric accessorywhen the load on said main unit is equal to or greater than saidprescribed threshold value.
 12. A method according to claim 8, whereinsaid second electric accessory includes at least an electricallyoperated air conditioner motor.
 13. A method according to claim 12,wherein said second electric accessory includes at least a portion,exclusive of said electrically operated air conditioner motor, connectedto said second power supply line through a third DC-to-DC converter forlowering a voltage, and also to a second electric storage device.
 14. Amethod according to claim 8, further comprising the steps of: detectinga failure of said DC-to-DC converter; and limiting the response ofelectric power output from said fuel cell if a failure of said DC-to-DCconverter is detected in said step of detecting.